Polyurethane high resilience foams are used in large amounts in widely diverse applications. One major commercial area is in the manufacture of seating components, for example furniture and seats and back rests for automobiles and other vehicles. In applications such as the latter, the seat cushions and backs, often containing metallic or polymer inserts for mounting in the vehicle, are molded in closed molds by pouring, spraying, or injecting a measured quantity of reactive, foamable polyurethane forming ingredients into a closed mold, or into an open mold which is subsequently closed. The normally closed cells of such foams are opened in situ, for example by the timed pressure release (TPR) method disclosed in the U.S. Pat. Nos. 4,579,700 and 4,717,518 or are opened by mechanical crushing, for example by hand crushing, roller crushing, and the like.
Molded polyurethane foam may be prepared using hot molding or cold molding techniques. In both methods, heated molds are generally used. In the preparation of hot molded foam, the polyurethane forming ingredients are introduced into the warm (c.a. 65.degree. C.) mold, and the entire mold placed in an oven (c.a. 180.degree. C.) to foam and cure. In cold molded foams, the warm mold is not placed in the oven, but the polyurethane foam simply allowed to cure in the mold. The foam may be demolded after achieving sufficient green strength, and may be post-cured at elevated temperature or by a more extended room temperature post cure.
Due to the problems associated with handling a very hot mold and demolding foam from such a mold, cold molding is preferred over hot molding. Moreover, as any foam post cure takes place outside the mold, the production rate from a given number of molds is increased with cold molding. The products obtained from these processes, however, have different physical properties, and the polyurethane forming ingredients, particularly the catalysts, surfactants, and most importantly, the polyether polyols, are different as between the two methods. Hot molded foam, for example, is generally harder than cold molded foam.
The physical properties of the foams are most important, and are generally set by the manufacturer. In seat cushions, for example, the hardness of the foam is selected to provide a comfortable seat. However, if the compressibility of the foam is too much, the occupant may feel the cushion springs or retainers. Furthermore, the properties of the foam must be maintainable over an extended period of use under varied conditions. Properties such as tensile and tear strength are also important, not only to prevent damage during use, but also to allow cushions to be installed into their frames and covers without damage. Early development of these properties during foam production is also necessary in order to successfully demold the molded foam part.
All foams exhibit some degree of compression set, a permanent or quasi-temporary loss of foam height after being compressed. A portion of the "set" may recover after time, but in general, a small amount of permanent set is introduced during early periods of use. Conditions of high temperature and humidity may aggravate the compression set, as well as other foam properties, due in part to plasticization of the polyurethane polymer by adsorbed water, but also, on occasion, by changes in polymer structure due to hydrolysis, reaction of unreacted isocyanate groups with atmospheric water, and disruption of hydrogen bonding between polar linkages present in the foam polymer.
To evaluate the effects of humidity on foams, the humid aging test (ASTM D2406) has been widely used. In this test, foams are artificially aged by placing the foam specimen in a steam autoclave for 5 hours at 120.degree. C. and 12 to 16 psig steam pressure, followed by drying at 70.degree. C. for 3 hours in a mechanically convected dry air oven. The foams are then allowed to equilibrate for 16 to 24 hours at 23.degree. C. and 50% relative humidity. Physical properties such as 50% compression set and 50% compression load deflection (CLD) loss are then measured. Foams are considered to be of high quality when their humid aged compression set and CLD values compare favorably with those of non-humid aged foam.
Recently, however, it has been found that foams which show satisfactory humid aged physical properties do not perform well in humid hot climates, for example, those characteristics of much of the Pacific Rim, the Mediterranean, and other tropic and subtropic environments. In many such cases, foams which exhibited satisfactory humid aged properties exhibited unsatisfactory loss in many properties, compression set and CLD particularly. Thus, the industry has recently developed more severe tests to evaluate foams.
"Wet Compression Sets" are one class of these more severe tests. Wet sets--like all compression set tests--specify a compression level and time, however, in wet set tests the foam is compressed at elevated temperature and humidity instead of elevated temperature only. One such test method described in K. Saotome et al., "The Improvement of Humidity Resistance in High Resilient Polyurethane Foam", J. CELL PLASTICS, May/June 1977, pp. 203-209, 1977, and in Toyota document BM7100G, Method 4.7.2, termed here as the "Japanese Wet Set", entails a 50% compression of a core sample for 22 hours at 50.degree. C. and 95% relative humidity. Japanese Wet Compression Set is measured after a 30 minute recovery at standard lab conditions (23.degree. C., 50% relative humidity). One other method, proposed by the European automotive manufacturer Renault, entails a 70% compression of a foam sample with skin for 22 hours at 40.degree. C. and 95% relative humidity. Renault Wet Compression Set is measured after a 15 minute recovery at standard lab conditions.
In both these tests, which are hereinafter referred to as "wet compression set" or "wet set," it has been found that foams which appear to have excellent humid aged properties have unsatisfactory wet set. This is particularly the case with cold molded foams, wherein the wet set may often be four times higher than the wet set produced by similar hot mold formulations.
U.S. Pat. No. 4,111,865 discloses foam formulations for high resiliency foams having improved humid aged properties, prepared using both hot molding and free rise techniques, these foams prepared from polyol mixtures containing a variety of 3.2 to 4.8 functional conventional and polymer polyols containing from 3 to 10 weight percent ethylene oxide as a cap, catalyzed by a mixture of tin and amine catalysts. However, the foam formulations disclosed by the '865 patent have a very narrow processing window, and are thus very difficult to manufacture on a commercial scale. Moreover, the humid aged properties appear to peak at ethylene oxide contents in the range of 3 to 5 weight percent, with foams prepared from polyols having a 7% ethylene oxide cap producing foams with increased humid aged compression set and load loss than foams prepared from 3 and 5 weight percent ethylene oxide capped polyols, respectively. The humid aged properties are traditional values, and not "wet set" values. The patentee states that polyols having greater than 10 weight percent oxyethylene caps result in polyurethane foams whose properties deteriorate to an excessive extent during humid aging.
It would be desirable to prepare reactive polyurethane foam formulations suitable for use in the cold molding of high resilience polyurethane foam wherein the resulting foam displays improved wet set properties. It would be further desirable to manufacture molded polyurethane foam articles exhibiting such improved properties.